Hindawi Canadian Respiratory Journal Volume 2020, Article ID 5852827, 16 pages https://doi.org/10.1155/2020/5852827

Review Article Postinfectious Bronchiolitis Obliterans in Children: Diagnostic Workup and Therapeutic Options: A Workshop Report

Silvija-PeraJerkic ,1 FolkeBrinkmann,2 AlistairCalder,3 AliciaCasey ,4 MeganDishop,5 Matthias Griese,6 Geoffrey Kurland,7 Mandy Niemitz,8 Sylvia Nyilas,9 Dirk Schramm,10 Ralf Schubert,1 Michael Tamm,11 Stefan Zielen ,1 and Martin Rosewich1

1Division for Allergy, Pneumology and Cystic Fibrosis, Department for Children and Adolescence, Goethe-University, Frankfurt/Main, Germany 2Department of Paediatric Pneumology, Children’s Hospital, Ruhr University of Bochum, Bochum, Germany 3Radiology Department, Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK 4Boston Children’s Hospital, Harvard Medical School, Boston, USA 5Division of Pathology and Laboratory Medicine, Phoenix Children’s Hospital, Phoenix, USA 6Department of Pediatric Pneumology, Dr. von Hauner Children’s Hospital, LMU Munich, German Center for Lung Research (DZL), Munich, Germany 7Division of Pediatric Pulmonary Medicine, Allergy and Immunology, Department of Pediatrics, University of Pittsburgh School of Medicine, Pittsburgh, USA 8University of Ulm Medical Centre, Clinic for Child and Adolescent Psychiatry/Psychotherapy, Ulm, Germany 9Division of Respiratory Medicine, Department of Pediatrics, University Children’s Hospital of Bern, University of Bern, Bern, Switzerland 10Department of General Pediatrics, University Children’s Hospital, Du¨sseldorf, Germany 11Department of Pulmonary Medicine, University Hospital Basel, Basel, Switzerland

Correspondence should be addressed to Silvija-Pera Jerkic; [email protected]

Received 27 August 2019; Revised 29 November 2019; Accepted 27 December 2019; Published 31 January 2020

Academic Editor: Motoshi Takao

Copyright © 2020 Silvija-Pera Jerkic et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Bronchiolitis obliterans (BO) is a rare, chronic form of obstructive lung disease, often initiated with injury of the bronchiolar epithelium followed by an inflammatory response and progressive fibrosis of small airways resulting in nonuniform luminal obliteration or narrowing. ,e term BO comprises a group of diseases with different underlying etiologies, courses, and characteristics. Among the better recognized inciting stimuli leading to BO are airway pathogens such as adenovirus and mycoplasma, which, in a small percentage of infected children, will result in progressive fixed airflow obstruction, an entity referred to as postinfectious bronchiolitis obliterans (PIBO). ,e present knowledge on BO in general is reasonably well de- veloped, in part because of the relatively high incidence in patients who have undergone lung transplantation or bone marrow transplant recipients who have had graft-versus-host disease in the posttransplant period. ,e cellular and molecular pathways involved in PIBO, while assumed to be similar, have not been adequately elucidated. Since 2016, an international consortium of experts with an interest in PIBO assembles on a regular basis in Geisenheim, Germany, to discuss key areas in PIBO which include diagnostic workup, treatment strategies, and research fields.

1. Introduction Lange described two cases of unknown origin [1]. Most early descriptions of BO were confined to case reports with Bronchiolitis obliterans (BO) is a rare, chronic form of autopsy findings; it was not until open lung biopsy became obstructive lung disease. ,e first report of BO in 1901 by a more common procedure possible to describe earlier 2 Canadian Respiratory Journal pathologic findings that suggested the cellular mecha- 3. Epidemiology nisms that ultimately led to BO. A review of the subject of BO in children summarized available studies suggesting PIBO is a rare disease but as there is no systematic case that BO begins with an injury of the bronchiolar epi- registration and as there are no national or international thelium followed by an inflammatory reaction which databases on PIBO, its incidence is unpredictable. ,e ep- progresses towards airway fibrosis and potential luminal idemiology is further hampered by a variable nomenclature. obliteration [2]. ,erefore, the frequency is largely unknown and possibly ,e term “bronchiolitis obliterans” likely describes a more frequent than expected, as many milder cases might common pathologic alteration of small airways following a remain undiagnosed. It is well known that PIBO is more variety of inciting diseases with different aetiologies and frequent among certain populations such as Argentinians, characteristics. ,e initial insult, the localized inflammatory Native Americans, and native Koreans [3–5] pointing to- response, and preexisting factors including nutritional status wards genetic factors playing a pivotal role in the initiation and genetic variants are felt to influence the process which or perpetuation of the process. finally leads to the observed pathology in small airways. BO following human stem cell transplantation (HSCT), HSCT- 4. Etiology BOS, complicated by graft-versus-host disease (GVHD) and BO following lung transplantation (LT), LT-BOS, have both BO is thought to be caused by an initial insult to the lower been extensively studied and are relatively well understood. airways [6]. However, there are three separate clinical en- Postinfectious BO (PIBO) in part because of its sporadic tities of BO. ,e severe injury to the lower respiratory tract appearance and low incidence has been much more difficult can be caused by either a pathogen such as adenovirus, to study. While there are study-based protocols for the influenza, measles, respiratory syncytial virus, and myco- evaluation and potential treatment of HSCT/GVHD and LT- plasma pneumonia (PIBO) or lung, heart (LT-BO), or bone related BO, the diagnosis and management of PIBO are not marrow transplantation (HSCT-BO) [3, 7]. In addition, as clear. A better understanding of the molecular and cellular some cases of BO were triggered by toxic gases [8], chronic mechanisms involved in the development of PIBO is needed aspiration [9], connective tissue diseases [10], and certain if more directed prevention and treatment strategies are to drugs [11]. ,e repair process of restoring the epithelium be uncovered. and microvasculature to its previous state is severely altered. Since 2016, an international consortium of experts ,is failure of resolution of the initial and ongoing consisting of pediatric pulmonologists, radiologists, pa- inflammation might be an important part of the disease thologists, physical therapists, psychologists, basic scien- process in PIBO. ,e influx of immune and inflammatory tists, and statisticians has gathered regularly for a cells and the proliferation of granulation tissue in the airways workshop on PIBO in Geisenheim, Germany. ,e work- lead to bronchial obstruction and to epithelial changes such shop has been complemented by affected families and as atrophy or hyperplasia [12]. representatives of the Foundation “Stiftung Starke Lunge” ,e etiology is used for clinical classification and is (http://www.starkelunge.de). ,e purpose of the work- important to guide the investigator to the diagnosis when shop has been to bring together international clinicians tachypnoea, wheezing, and hypoxaemia persist for at least 2 and researchers and to exchange and discuss new findings months after a causative event. Accordingly, most of the BO and current research data in the field of PIBO. ,e cases can be classified into postinfectious and posttransplant multidisciplinary workshop presentations have included BO [2]. definitions, epidemiology, etiology, and clinical courses of PIBO. ,e focus of the attendees has been set on specific 5. Clinical Course key areas which included diagnostic workup, treatment strategies, and research fields. In conclusion, future per- ,e diagnosis of PIBO is usually late as the initial presen- spectives and joint research goals were being discussed tation shows a large overlap with much more frequent and distributed. airway diseases, and many causative agents have been de- ,e following article serves as the first official workshop scribed. By the time of diagnosis, the airway disease is summary of an international consortium of experts in frequently advanced and irreversible fibrotic changes and PIBO. airway obliteration have been established making the treatment difficult and often unsuccessful. As disease pro- gression or disease stages are not defined yet, the recom- 2. Definition mendation of an appropriate treatment is further hindered by changes of inflammatory patterns over time. In the past, there have been several attempts to classify While the clinical course of BOS after HSCT has been the condition PIBO. However, none of the present def- described in the literature [13, 14], there are no existing initions are widely accepted. PIBO is a process charac- reviews summarizing the clinical course of PIBO. ,e terised by persistent airway obstruction with functional prognosis of PIBO seems to have a better outcome than and radiological evidence of small airway involvement HSCT-BOS and LT-BOS, respectively. However, the course that is in general unresponsive to bronchodilator varies in each case and therefore cannot be predicted. treatment. Empirical data reveals three different patterns of progression Canadian Respiratory Journal 3

after an initial insult which were (a) an imperceptible start FEV1 and slow but steady deterioration, (b) an initially rapid (a) deterioration followed by a stable state, and (c) a rapid (b) deterioration (Figure 1). (c) 6. Diagnostic Workup t ,e diagnosis of PIBO is usually made by a combination of medical history, clinical findings, lung function testing, and Figure 1: Patterns of progression in BO. imaging, although biopsy and histopathology remain the diagnostic gold standard. diagnosis and management of BOS post-LT. It defines BOS as a small airway disease diagnosed with a persistent 6.1. Clinical Findings. PIBO is diagnosed by clinical criteria decrease in FEV1 of at least 10–20% compared to the mean describing symptoms such as tachypnoea, cough, wheezing, of the two best postoperative values after lung trans- exercise intolerance, and hypoxaemia persisting for at least 6 plantation in the absence of any other identifiable causes weeks after severe bronchiolitis or pneumonia with respi- [18]. Because biopsy is invasive and associated with risk of ratory insufficiency [2]. ,e diagnosis is often associated bleeding and further complications, the National Institute with chronic obstructive lung disease after a severe viral of Health (NIH) Consensus proposed new criteria for illness in childhood [2, 3]. diagnosis and scoring the severity of chronic GVHD of the In case of a suggestive medical history of severe re- lungs pointing towards BOS [19]. ,e clinical diagnosis of spiratory infection in a previously healthy patient, the BOS post-HSCT is made when FEV1/VC < 0.7, physical examination is not very helpful in diagnosing FEV1 < 75% of pred. with >10% decline over less than 2 PIBO. ,ere are nonspecific signs which point towards years, absence of infection, and evidence of air trapping by PIBO such as crackles, wheezing on chest auscultation, and CTor lung function testing [19]. However, diagnosing and hyperinflation. scoring the severity of BOS remains challenging due to limited understanding of pathophysiology, and current guidelines for BOS point out the need to find new sensitive 6.2. Lung Function Testing lung function indices to detect early small airway im- pairment [18]. 6.2.1. Spirometry. Classical spirometry is a commonly used In the last decade, the usability of new inert gas washout technique which detects large airway obstruction. However, measurements in different lung diseases gained worldwide it is insensitive in small airway disease and in gradual disease attention. Nitrogen multiple breath washout (N2-MBW) is a progression. Pulmonary function tests usually document an feasible and sensitive tool to detect early small airway im- obstructive impairment; however, in early stages, these tests pairment in chronic lung diseases in children and adults may be normal. [20–24]. ,e main outcomes detect global (lung clearance Lung function testing in patients with PIBO presents index: LCI) as well as acinar (Sacin) ventilation inhomo- with typical patterns. ,e spirometry shows an irreversible geneity. Previous studies showed that the lung clearance or fixed obstructive flow-volume curve with decreased index (LCI) is already abnormal in the majority of patients forced expiratory volume (FEV ), a reduced Tiffeneau index 1 with cystic fibrosis (CF), primary ciliary dyskinesia (PCD), (FEV /VC), and end-expiratory flow (MEF25). On body 1 and chronic obstructive pulmonary disease (COPD) even plethysmography, hyperinflation and air trapping are in- though spirometry indices are still in the normal range dicated by an increased residual volume (RV) and an in- [20, 24–27]. ,e N2-MBW derived Sacin correlates closely creased functional residual capacity (RV/TLC) [5, 15]. with impaired ventilation in the distal lung compartment, Typically, there is either no or little response to broncho- the acinus [28], and is highly elevated in adults and children dilatation [16]. Patients with PIBO tend to have milder with small airway disease [29–31]. diseases than those with BOS following bone marrow Lung clearance index (LCI) measured by multiple breath transplantation [3]. (Figure 2) A cohort study by Colom et al. washout (MBW) is likely to be of benefit in PIBO. ,e reported severely impaired pulmonary function among spirometry aids to diagnose PIBO in children, and it detects children with PIBO after long-term follow-up [17]. Dis- irreversible bronchiolar obstruction. However, spirometry proportional growth of lung parenchyma in airways of af- in very young children with PIBO is not feasible due to fected children was a proposed explanation. limited cooperation. A recent prospective study by Kim et al. evaluated the utility of LCI assessing the degree of small 6.2.2. Multiple Breath Washout. Spirometry primarily airway obstruction in children with PIBO [32]. Twenty measures obstruction in the larger airways; however, it is a infants diagnosed with PIBO underwent pulmonary lung generally insensitive detector of small airway obstruction. function tests, MBW, and chest CT. ,e concordance in- A joint task force formed by the International Society for dices of LCI were significantly correlated with the air- Heart and Lung Transplantation (ISHLT), the American trapping lung volume percentage from CT which suggests ,oracic Society (ATS), and the European Respiratory that LCI is a feasible, complimentary tool for assessing Society (ERS) has published a practice guideline of children with PIBO. 4 Canadian Respiratory Journal

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3 F/V ex Vol (L) 3 TLC 2

2 1 Fluss (L/s) FRCole 1 0 1 0.5 1.0 1.5 2.0 RV Vol (L) 0 1 Soll Ist 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Zeit (min)

(a) (b)

Figure 2: Lung function test in PIBO.

A study in thirty-seven pediatric patients >100 days after requires minimal cooperation and does not modify the lung transplantation showed highly elevated LCI values in airway smooth muscle tone. For preschool children and comparison with FEV1/FVC and a poor agreement between children unable to successfully perform spirometry, it can both parameters [33, 34]. In line with this, another study become difficult to understand and perform forced expi- which enrolled 15 patients, 5 of whom already had a di- ratory manoeuvres, and therefore, spirometry results may agnosis of chronic lung allograft dysfunction (CLAD), not be evaluable. A study by Evans et al. showed that re- showed that the LCI identified lung allograft dysfunction in spiratory impedance outcomes were significantly impaired more patients than the use of standardized spirometric in three groups of children with asthma, CF, and children measures [35]. born preterm, respectively, compared to healthy controls ,ese results indicate as well that a combination of [37]. However, their findings also showed that the utility of different lung function parameters may be more predictive specific FOT outcomes is dependent on the respiratory of disease course and prognosis than any single parameter disease being assessed, and therefore, a more detailed dis- and that LCI should be established as a useful comple- ease-specific approach to interpreting FOT would be es- mentary parameter in assessing patients with PIBO. Further sential. ,ere are no studies looking at the feasibility of FOT longitudinal studies will show if inert gas washout indices in PIBO: however, it would be an interesting tool to look at can be used as an early predictive outcome to detect early in preschool children patients with PIBO. Further studies are PIBO. At present, LCI, which is the main outcome pa- needed to demonstrate whether FOT is a useful comple- rameter from the N2-MBW, detects global ventilation in- mentary technique in identifying and assessing young pe- homogeneity with good sensitivity. In order to analyse the diatric patients with PIBO. specificity, it is recommendable to measure N2-MBW during clinical routine accompanied by commonly used 7. Imaging lung function test on a regular basis on one to three monthly follow-ups. Hence, N2-MBW indices could be used as pa- 7.1. Pathobiological Basis of CT Findings in PIBO. ,e rameters to predict patient’s outcome. Furthermore, N2- principal finding in PIBO is variation in the density of al- MBW could be used for clinical research to assess the veolar lung tissue, termed “mosaic attenuation.” Diseased treatment effects of therapeutic interventions in PIBO. lung shows lower density through two principal mecha- nisms, alveolar hyperinflation and hypoxic vasoconstriction [38]. Histological studies suggest that hyperinflation can 6.2.3. Forced Oscillation Technique. ,e forced oscillation occur with both complete and incomplete bronchiolar ob- technique (FOT) is a noninvasive method with which to struction; hyperinflation in the presence of complete measure respiratory mechanics [36]. ,e ERS formed a joint bronchiolar obstruction must reflect a “check-valve” ERS task force which led to publication of a practice mechanism of collateral air circulation [39]. Hypoxic va- guideline on clinical application of FOT in 2003 [36]. soconstriction leads to redistribution of blood flow to Especially for the pediatric population, FOT has the “healthy” lung. When bronchiolar disease is extensive, a advantage over spirometry that it does not require the larger volume of blood will be redistributed to a smaller performance of respiratory manoeuvres due to the small- volume of healthy lung, which will become denser as a result, amplitude pressure oscillations superimposed on the normal demonstrating ground-glass attenuation. ,us, in extensive breathing. Moreover, in contrast with spirometry where a disease, mosaic attenuation is accentuated (Figures 3(a) and deep inspiration is needed, forced oscillation technique 3(b)). Canadian Respiratory Journal 5

(a) (b)

Figure 3: CT chest in PIBO.

7.2. CT Techniques for Evaluating PIBO. ,ere are a number Another traditional aspect of CT scanning for PIBO and of decisions to be made in selecting a CT technique in this other forms of diffuse lung disease has been the use of setting. interrupted as opposed to contiguous volume acquisitions. Adult studies suggest that performing inspiratory CT ,e purpose of interrupted sections (typically 1 mm sections only will miss some cases of bronchiolitis obliterans and that at 10 mm intervals) is to reduce radiation dose. However, the adding in expiratory sections increases sensitivity [40, 41]. reduction in information can lead to reduced sensitivity and Interestingly, those with abnormalities on expiratory scans precision [44], and, as other radiation dose reduction had milder disease. Obtaining expiratory sections reliably in techniques are available, this technique is much less fre- young children (under age 6) generally requires general quently performed today. anaesthesia. An alternative for “free-breathing” scans is to ,e use of intravenous contrast is also discussed. In perform lateral decubitus acquisitions, with the dependent general, this can be avoided in the initial diagnosis of PIBO, lung acting as the “expiratory” lung. Although there is ev- avoiding the need for cannula placement. It is unlikely that idence that this may be effective [42], workshop attendees’ IV contrast either enhances or interferes with the diagnosis experience is that this is not always so in clinical practice. of PIBO, but its use may be important in severe or atypical Postprocessing techniques during image interpretation disease, for example, in children with pulmonary hyper- can enhance the detection of mosaic attenuation. ,e use of tension, those being considered for lung transplantation, or minimal intensity projection (minIP) is one such method those who may have lymphadenopathy. which may be effective [43]. A critical factor is the selection Radiation dose from CT scanning is a major limitation of window settings. Lung images are typically evaluated on its use. Fortunately, a range of modern innovations has using a wide window width (1200–1500), but mosaic at- seen the dose from CT fall dramatically, and where his- tenuation can be much more easily detected with a narrower torically, radiation doses were in the order of 10 mSv, and it window width (∼300). is now possible to scan at doses of well under 1 mSv. For Historically, CT has been difficult to perform in young comparison, annual background radiation dose in the UK is children. Whilst infants could sometimes be scanned asleep approximately 2.7 mSv. ,e risks of such level of radiation after feeding (“feed and wrap”), children between the ages of are unknown, and a commonly used approach applying the 3 months and 4–6 years generally required general anaes- so-called “linear no-threshold model” suggests that a thesia to perform thoracic CT imaging. However, the advent 0.5 mSv exposure at age 1 would convey a lifetime excess of ultra-high-speed scanning allows a much lower utilisation cancer risk of 1 : 20000 [45]; this is set against a background of general anaesthesia. Some modern scanners can perform lifetime cancer risk for children born today approaching 1 : scans of the entire thorax in approximately 0.3 seconds. ,is 2. However, increasingly, it is being suggested that low can almost eliminate artefacts from motion and breathing. radiation exposure may not carry any excess risk at all [46]. When combined with gentle immobilisation, the use of Potential alternatives to CT as an advanced imaging tool for general anaesthesia for chest CT scanning in 1- to –6-years- PIBO include various forms of MRI scan (see below). olds in one centre practicing these methods has fallen from 50% to 10%. A downside of avoiding general anaesthesia is the inability to select the respiratory phase, but this can be 7.3. Ae Role of CTin the Modern Diagnosis of PIBO. CTplays ameliorated by the techniques described above. a central role in the diagnosis of PIBO; the finding of mosaic 6 Canadian Respiratory Journal attenuation on CT is the strongest predictor of PIBO (along available and in many centres is only available on a research with a typical clinical history) [47]. basis; finally, and most importantly, MRI almost always PIBO typically manifests a clear clinical history, and as requires general anaesthesia in those under the age of 5-6 such, differential diagnoses often do not need to be con- years, whilst PIBO tends to have its first onset at age 1-2 sidered. However, if disease onset is insidious or if there are years. other complicating features, then other entities are some- times considered, including cystic fibrosis, other causes of severe bronchiectasis and severe asthma. 7.5. Concluding Comments. CT has a clear, logical role in In general, these are best diagnosed by nonradiological the diagnosis of PIBO. However, the scientific evidence means, as there can be considerable overlap in the CT base for this is somewhat limited and in large part derives findings. An adult study suggested that mosaic attenuation from studies in adults with noninfectious etiologies of on inspiratory CT was the best distinguishing factor of adult PIBO. ,ere are a number of open questions in this field. BO (of undefined etiology) from severe asthma [48]; al- Critically, imaging for PIBO tends to be performed ternatively, a study in children suggested that the presence of sometime after the initial pulmonary insult (typically ad- bronchial dilatation/bronchiectasis, present in 88% of enovirus). It is likely that the main window for modulating children with PIBO, but only 29% with severe asthma, was a the outcome in PIBO is earlier than this. An ability to better distinguishing criterion [49]. Another possible role for distinguish uncomplicated from complicated adenovirus CT in PIBO is as a marker of disease severity. Several studies pneumonia at the time of initial infection is needed if future have demonstrated correlation between various CT mea- interventions are to be correctly targeted (e.g., immuno- sures of disease severity and lung function measures in BO of modulation); it is unknown whether imaging might play a mixed etiologies [50–52], but none of these suggest com- role in this. Outcomes in PIBO are variable, with about 20% pelling support for CT as a severity measure. Applying of cases progressing to a severe or fatal outcome and 20% automated and semiautomated approaches is likely to en- making a full recovery; whether imaging can predict these hance the precision of CT measures of lung severity. Two outcomes is currently unknown. Finally, if early inter- such approaches are (1) measuring the volume of “air- vention studies are to be carried out in the future, CT may trapped” lung using a “density mask,” an adjustable seg- be proposed as a surrogate outcome measure; however, mentation threshold [32, 53], and (2) parametric response further validation for such a role would need to be carried mapping, whereby voxels on paired inspiratory and expi- out. MRI has some advantages over CT as an imaging tool ratory scans are matched and scored according to the for small airways disease, but is yet to be validated and is densities on each [54]. often impractical, in the younger children with post- infectious BO.

7.4. MRI in PIBO. MRI imaging of the lungs has undergone 8. Histopathology significant developments in recent years and has attracted interest for pediatric imaging in particular in view of the lack ,e term bronchiolitis obliterans as a histopathological of ionising radiation. Structural lung imaging with MRI is entity was classified with two distinct types of bronchiolar challenging owing to low signal-to-noise ratio and the involvement. It includes the constrictive type with peri- poorer spatial resolution of MRI compared with CT. Newer bronchial fibrosis and the proliferative type with airway sequences however, particularly ultra-short echo time (UTE) obstruction by intraluminal polyps of inflammatory sequences, have been shown to produce excellent quality granulation tissues [60]. images in a range of lung pathologies, including airway Lung biopsies have been considered the gold standard disease such as cystic fibrosis [55]. MRI also has the added for diagnosing BO. Typically, these biopsies show a pro- advantage of greater capacity for functional imaging. MRI gressive inflammatory response with elements of tissue ventilation imaging of the lungs using hyperpolarised gases remodelling, fibrosis of the small airways, and airway ob- (e.g., He3 Xe129) has been shown to be exquisitely sensitive struction. However, due to the heterogeneous distribution of for small airways disease in CF [56] and would appear to be a airway involvement throughout the lung parenchyma and logical choice for PIBO. However, these techniques are not the variability of chronic inflammation among patients, widely available, requiring bespoke equipment to generate these biopsies can lead to sampling error which reduces the the gas agents, and specialised coils and MRI systems. Al- sensitivity. In addition, there are only some similarities at the ternative approaches to MRI ventilation imaging include initial airway injury and early healing phase such as lym- using 100% oxygen and mapping its effect on T1 signal [57] phocytic inflammation in patients with PIBO, LT-BO, and and using multivolume MRI sequences to explore changes in HSCT-BO. ,e end-stage pathology reveals the same fibrotic MRI signal intensity over the respiratory cycle and thereby airways with airway dropout. derive ventilation maps [58, 59]. ,ere are several drawbacks In this regard, BO remains a joint diagnosis considering to MRI: firstly, it has not been systematically evaluated in the clinical, radiological, and histopathological features. setting of PIBO as opposed to other forms of chronic airway Typical lesions are presented in the following figure disease (e.g., chronic lung allograft dysfunction and CF) or (Figure 4). in younger children. Secondly, lung imaging generally re- Figure 4(a) shows typical lesions of obliterative bron- quires additional sequences, agents, or equipment not widely chiolitis, demonstrating a preserved smooth muscle wall Canadian Respiratory Journal 7

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2. 3. 2.

(a) (b)

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1. 2.

(c) (d)

Figure 4: (a) Haematoxylin and eosin staining (10x original magnification). (1) Cholesterol clefts; (2) smooth muscles; (3) luminal obliteration; (4) scattered lymphocytes. (b) Movat pentachrome (10x original magnification). (1) Unpaired artery branch; (2) obliterated bronchiole. (c) Haematoxylin and eosin staining (20x original magnification). (1) Constrictive bronchiolitis with sub- epithelial fibrosis and cellular infiltrates. (d) Haematoxylin and eosin staining (10x original magnification). (1) Cartilage island; (2) fibromuscular scar. with luminal obliteration by fibroconnective tissue and 9. Treatment Options scattered lymphocytes. ,ere is no residual mucosa. Peri- bronchiolar accumulation of macrophages and cholesterol Since PIBO is a rare chronic irreversible obstructive lung clefts are common secondary findings resulting from small disease, treatment options have not been clearly defined and chronic airway obstruction and may be a clue to the diag- there are different strategies between centers. Most of the nosis at low power. current knowledge comes from studies looking at patients Figure 4(b) shows at screening magnification the finding with BOS either post-LT or post-HSCT [61]. of an “unpaired” pulmonary artery branch which allows ,e treatment of PIBO is empirical, and there is no recognition of an adjacent obliterated bronchiole. accepted treatment protocol. However, there are a few In Figure 4(c), constrictive bronchiolitis can be seen randomized placebo-controlled trials in BOS. Most of the which indicates narrowing of a bronchiole lumen by varying reports published on treatment options suffer from small degrees of subepithelial fibrosis, with or without cellular patient numbers, absence of controls, and diverse patient infiltrate. ,is example shows mild constrictive bronchiolitis groups at the start of therapy. as part of the spectrum of pathology in bronchiolitis In general, the treatment for PIBO should be a com- obliterans syndrome. bination of optimal supportive care and anti-inflammatory Figure 4(d) is an example that obliteration of a bronchus therapy to impair lymphocyte proliferation and activation is recognized by an island of cartilage adjacent to a fibro- since inflammation plays an important role in the patho- muscular scar. “Bronchiolitis obliterans” may be seen fol- genesis of PIBO [62] (Table 1). Nevertheless, it is common lowing severe necrotizing respiratory viral infection, such as consensus that before systemic anti-inflammatory treatment influenza, and is a common end-stage manifestation of is given, a diagnostic workup including bronchoscopy with Stevens–Johnson syndrome. bronchoalveolar lavage (BAL) should be performed 8 Canadian Respiratory Journal

Table 1: Treatment options in PIBO. Anti-inflammatory therapy Supportive care (i) Systemic corticosteroid (i) Supplemental O2 (ii) Azithromycin (ii) Nutritional support (iii) Combination-therapy: FAM (fluticasone/azithromycin/montelukast) (iii) Immunization (influenza/pneumococcal) (iv) Immunglobulin substitution (iv) Avoid cigarette smoke (v) Steroid sparing anti-inflammatory agent (v) Airway clearance if bronchiectasis (hypertonic saline) (vi) Tumor necrosis factor inhibitor (vi) Bronchodilators if responsive (vii) Rescue therapy (extracorporeal photopheresis) (vii) Exercise therapy/pulmonary rehabilitation thoroughly to rule out persistent infections with viral, infections, a noncorticosteroid treatment for long-term fungal, and bacterial pathogens. management is recommended. It is important to note that a study analysing sleep- disordered breathing in children with PIBO showed that the risk of nocturnal hypoxia was increased in patients with 9.2. Efficacy of Azithromycin. It is well known that corti- PIBO and it was correlated with the severity of lung disease costeroids do not target neutrophilic airway inflammation determined by pulmonary function test. While the duration efficiently. In contrast, azithromycin has been effective in of central apnoeas was shorter, they were more prone to controlling neutrophilic inflammation and improving lung desaturations [63]. function in various diseases such as diffuse panbronchiolitis, cystic fibrosis COPD, and BOS post-lung transplantation [69–71]. A randomised controlled trial of azithromycin 9.1. EfficacyofSteroids. In dependence on the clinical course, therapy in BOS post-lung transplantation by Corris et al. inhaled and systemic corticosteroids are used to counteract improved FEV1 in patients with BOS post-LT and appeared the inflammatory component. Ideally, corticosteroids superior to placebo [72]. A pilot study of six patients by should be given early during the developing disease process Gerhard et al. showed that azithromycin improved FEV1 in and before airway fibrosis is established [15]. It is common five patients with BOS post-LTwith a mean increase of 17.1% agreement that the approach of choice is pulse steroid predicted over a 4-month period [73]. Verleden revealed therapy with intravenous methylprednisolone 10–30 mg/kg that azithromycin improved FEV1 and decreased airway for 3 consecutive days and repeated monthly for 3–6 months neutrophilia and IL-8 levels in patients with reversible as it is used for childhood interstitial lung disease. Oral neutrophilic inflammation in post-LT-BOS [71]. ,e exact corticosteroids and an elongated course of systemic corti- mechanism by which azithromycin downregulates the host costeroid should be avoided, since this is associated with inflammatory response is largely unknown. Different severe side effects and complications like bone fractures or mechanisms have been described so far such as reduced mortality from infections. Some centers report that during microbiologic burden, decreased numbers of airway neu- the vulnerable time of the corticosteroid pulses, they would trophils, suppression of IL-8, and other neutrophil che- give intravenous immunoglobulins as supportive treatment motactic cytokines and interference with neutrophilic to prevent infections. However, there are no published data function. available to demonstrate the benefit of such immunoglob- Apart from reports with significant clinical response to ulin treatment. azithromycin, there are studies where azithromycin therapy ,e available data on corticosteroid efficacy in PIBO are did not show any improvement in lung function. A systemic rather limited (Table 2). Tomikawa et al. studied 40 patients review and meta-analysis on the impact of azithromycin on who were diagnosed with PIBO and who were treated with change in FEV1 shows that there was no sufficient evidence methylprednisolone pulse therapy in monthly cycles and to support or refute the use of azithromycin in the treatment additional inhaled corticosteroids for the follow-up period of patients who develop BOS post-HSCT [74]. [67]. ,e frequency of wheezing exacerbations after 24 With regard to azithromycin as treatment for PIBO, months of methylprednisolone pulse therapy and frequency the literature is very scarce (Table 3). A study by Li et al. of hospitalization after 18 months were significantly reduced looked into 42 cases of children diagnosed with PIBO who and oxygen saturations improved. Yoon et al. evaluated the revealed some benefit of being treated by a combination of CT features that predict responsiveness to methylprednis- systemic corticosteroids and oral azithromycin [64]. Al- olone pulse therapy. ,e authors concluded that children though there are no RCTs in children with PIBO, oral with PIBO may respond favorably to pulse therapy when azithromycin 10 mg/kg given three times weekly is rec- pretreatment CT indicates bronchial wall thickening [68]. ommended on the basis of studies in other obstructive Although corticosteroids are effective in acute deterioration diseases [15]. of PIBO and in acute graft-versus-host disease (GVHD) by Although there are no RCTs in children with PIBO and improving lung function and exercise capacity, the long- since azithromycin is well tolerated, oral azithromycin term benefit is largely unknown. In view of the unclear effect 10 mg/kg given three times weekly is recommended as long- of corticosteroids upon the long-term clinical outcome, their term management on the basis of studies in other ob- substantial toxicities, and a high risk for serious and fatal structive diseases [15]. Canadian Respiratory Journal 9

Table 2: Corticosteroids in PIBO. Study (year) Chen et. al. (2012) Chinese Wang et. al. (2015) Experimental & Li et. al. (2014) BMC Pediatrics [64] journal Journal of Pediatrics [65] ,erapeutic Medicine [66] Number 26 children 42 children 16 children Comparative No control group/no No control group/no placebo No control group/no placebo design placebo Prospective cohort Retrospective cohort Study design Retrospective cohort Oral prednisone Oral prednisolone (1.5 mg/kg/day with weaning for 6–9 mo) (1 mg/kg/day with dose reduction of Treatment and azithromycin (5 mg/kg for 3 d/wk for Oral steroid and 1.25 mg/month) and azithromycin (7.5 mg/ regime 6 mo) azithromycin kg twice weekly) Some also had i.v. steroids during acute Variable treatment duration phase Assessment of Effective (subjective/objective measures Clinical symptoms and HRCT findings outcome PFT <10% decline) or ineffective Effectiveness Effective in 86% at 6 months 10 of 16 improved

Table 3: Potential biomarkers in BO. Parameter Entity Serum BAL/sputum Evidence Reference PIBO Neutrophils — Increased A Eckrich et al. [75] BOS CRP BOS Increased Increased A Vos et al. [76] PIBO IL-1β — Increased A Rosewich et al. [62] BOS PIBO IL-6 — Increased A Rosewich et al. [62] BOS IL-8 PIBO — Increased A Koh et al. [77] YKL-40 PIBO Increased — B Jang et al. [78] KL-6 BOS Increased — B Ohshimo et al. [79] Surfactant protein D BOS Decreased — B Nakane et al. [80] Metalloproteases 8 + 9 BOS Normal Increased B Taghavi et al. [81] IL-17/IL23 BOS — Increased B Vanaudenaerde et al. [82] CD8 T BOS Increased Increased B Hodge et al. [83] NK cells BOS Increased Increased B Hodge et al. [83] CXCL9 BOS — Increased B Shino et al. [84] miRNA-2/miRNA-155 BOS Increased — Budding et al. [85] MIP-1α/CCL-3 BOS — Increased B Verleden et al. [86] A: described by several studies; B: few reports.

9.3. Efficacy of Fluticasone, Azithromycin, and Montelukast PIBO. Although this treatment option is safe, no formal (FAM). In a pilot study, montelukast has been tried in trials have been conducted yet. fibroproliferative post-lung transplant BOS and showed slowing of decline in FEV1 [87]. However, a recent trial by Ruttens et al. showed no additional survival benefit with 9.4. Efficacy of Bronchodilator Treatment. Chronic ob- montelukast compared with placebo [88]. Several studies structive airway disease and hyperinflation play an impor- reported that a combination of inhaled fluticasone, azi- tant role in the pathophysiology of PIBO. Typically, these thromycin, and montelukast (FAM) could be an effective patients do not show a significant postbronchodilator re- treatment in patients with BOS [89]. sponse after inhaling 400–600 μg salbutamol. In the recent Recently, a phase II, open-label, multicenter study study of Rosewich et al., 5 out of 20 patients showed evaluated the treatment effect of FAM in combination with a postbronchodilator changes consistent with reversibility steroid pulse at the start of therapy. ,e primary endpoint based on FEV1 (12% increments and 200 ml) [62]. Whether was treatment failure, defined as 10% or greater FEV1% this is due to concomitant allergies or asthma or whether it is decline at 3 months [90]. At 3 months, only 6% vs. 40% a symptom of a distinctive subtype of PIBO is difficult to say. historical controls (p < 0.001) had treatment failure. ,ese Two studies reported that patients do favourably respond to data suggest that FAM was well tolerated and that treatment long-acting muscarinic receptor antagonists (LAMAs). In- with FAM and steroid pulse may halt pulmonary decline in terestingly the placebo-controlled trial of Teixeira demon- newly diagnosed BOS post-HSCT. strated a meaningful improvement after tiotropium alone in ,e efficacy of FAM in PIBO is not known. Single centers PIBO [91] and after tiotropium plus salbuterol in BOS post- have reported to use FAM as a combination therapy in HSCT [92]. In a multicentre study with adult patients by 10 Canadian Respiratory Journal

Bergeron et al., it was revealed that there was a significant development of new classes of therapeutic compounds has improvement in FEV1 in patients with mild to severe BOS been described recently [103, 104]. post-HSCT after a 6-months trial of budesonide/formoterol In view of these findings, the assumption has been raised [93]. whether miRNAs could play an important role in the process of inflammation and development of fibrosis in PIBO. Further research is required to evaluate the role of miRNA in 9.5. Potential Future Treatment Options. ,ere has been PIBO. some debate about future treatment options. However, these options are still being assessed and further research studies are required. ,ere are some recent preclinical and clinical 10.2. Predictive Biomarkers. Distinctive inflammatory pro- studies of lung diseases using mesenchymal stem/stromal files have been sought to improve our understanding of cells (MSCs). MSCs have shown immunomodulatory and ongoing inflammation and remodelling in airway diseases anti-inflammatory properties and have proven some efficacy such as cystic fibrosis and asthma [105–108]. in various lung injury models in animals. However, they can Equivalent data for PIBO are lacking, in part because of also have profibrotic effects. ,ere are multiple studies with an absence of reproducible animal models for which po- animal models using MSCs to treat BOS [94–98]. Cao et al. tential biomarkers can be determined and in part because of showed that in seven patients with BOS and respiratory the rarity of PIBO following life-threatening infections. failure, six of them presented with lung function im- Adenoviral lower respiratory infections are a recognized risk provement 6 months after they were given i.v. MSCs for 4 factor for the development of PIBO, yet the incidence of this weeks along with budesonide, salmeterol, and methyl- complication in children with documented adenovirus is prednisolone [99]. quite variable ranging from less than 5% [109, 110] to greater Extracorporeal photopheresis (ECP) [100] and total than 25% in certain populations [111, 112]. ,e reason for lymphoid irradiation (TLI) [101] have been used to treat this wide difference in incidence remains unclear, although BOS post-LT with variable efficacy. ECP did show reduction variations in viral prevalence and virulence have been of lung function decline and was well tolerated in a single suggested as potential explanations [113]. center with 10 years of experience [102]. ,e search for biomarkers that potentially could be ,e treatment of BOS post-HSCTremains difficult. It has useful in predicting PIBO following adenoviral or myco- been reported that infliximab, a monoclonal antibody di- plasma pulmonary infections has centred on studies of rected against tumor necrosis factor-α (TNF-α), has been patients with life-threatening respiratory complications of successfully used in a child with BOS post-HSCT. Further these pathogens [62, 114, 115]. Potential biomarkers relevant treatments including inhaled cyclosporine and clotrimazole to the underlying disease process in PIBO are listed in might become an option; however, additional trials and Table 3. studies need to be conducted. Based on these studies as well as investigations of animal models of BOS following lung or stem cell transplantation, 10. New Research Fields there are felt to be several factors involved in the devel- opment of PIBO: (a) the virulence of the inciting pathogen ,e current knowledge of the underlying mechanisms of itself, possibly serotype-specific, causing airway epithelial PIBO is scarce and treatment is challenging. Multicentre injury; (b) cytokines including IL-6, IL-8, and TNF- α; (c) research is essential to understand the exact mechanisms of other molecular components which are upregulated locally; PIBO, be able to identify biomarkers and clarify monitoring and (d) cellular components, either locally “in situ” or di- and treatment. As PIBO affects mainly children, further rected by cytokines and other mechanisms to the region of research and clinical trials need to include pediatric patients. airway damage. ,e international consortium of clinical and scientific ,rough an orchestrated interaction, the airway epi- experts in PIBO has made a multidisciplinary effort to set up thelial damage, rather than repairing it in a normal fashion new, collaborative research approaches. ,e focus was set on with a normal epithelial layer, is largely replaced by T cells the following three areas of investigation. and neutrophils, with subsequent matrix degradation, col- lagen deposition, and fibroblast stimulation, ultimately leading to the airway fibrosis characteristic of BO [6]. 10.1. MicroRNA. New techniques such as next-generation ,e lack of a good animal model of PIBO hampers the sequencing (NGS) for sequencing large amounts of data efforts to determine which of the many potential biomarkers have fundamentally changed the viewpoint on scientific and would serve as a reasonable potential target in human trials. diagnostic questions. MicroRNAs (miRNAs) play an im- An example of the difficulties of a possible animal model is portant role in the pathogenesis of numbers of respiratory that of mouse adenovirus-1 (MAV-1) respiratory infections diseases such as asthma, CF, idiopathic interstitial pneu- in neonatal mice. ,e extensive and careful work of monia, and COPD. miRNAs are small noncoding RNAs that Weinberg and his colleagues has shown that neonatal mice control gene expression posttranscriptionally by inhibiting thus infected have higher viral loads than adult mice. In translation, and thus, they play a central role in the epige- addition, infected neonatal mice have impaired recruitment netic modification of gene expression. In addition, their of inflammatory cells compared with adult mice. Yet, per- promising potential as biomarkers and basis for the haps paradoxically, viral clearance is unimpaired in neonatal Canadian Respiratory Journal 11

Table 4: Patient’s perspectives. Diagnosis Where do I get the right diagnosis? Where can I find help and who can I approach if I realize that my child remains unwell? What does it do to my child and our family when the correct diagnosis is established? Where can I get any specific information? What is the long-term prognosis? Will my child die? Overall care What kind of therapy is available for my child and is there a cure? Does my child need regular physiotherapy and is any specific physiotherapy required? Does my child need psychological support and where would it be available? Do we have to carry all the costs for medical and supportive therapy? Is there any support for travel costs? Family-related How often do we need to see a specialised center? How far do we have to drive to get to a center? Will I need to take off work for every appointment? How can I talk to my child about the illness? What impact does a chronic disease have to family life? Where can we go if we do not manage it? What impact has the illness to the siblings? Will there be any support for them? Harm What side effects do the treatments have? What implications does a chronic disease have for my child’s future life? Will my child be able to live on his own and will my child be able to live on own responsibility? Does the diagnosis limit my child in his career choice? Can my child undergo a normal teenage life with sport, leisure, party, stress? mice [116]. ,ere are multiple interactive cytokines that are weak points including low sensitivity and specificity, and involved in viral clearance as well as inflammatory cell re- often it is difficult to distinguish between colonization and cruitment in this animal model [117, 118]. infection. From the onset of the diagnosis, regular PIBO Further, neonatal mice surviving MAV-1 respiratory microbiome analysis would be needed to comprehend infection do not show histologic changes suggestive of microbiome-driven pathophysiology and inflammation in bronchiolitis obliterans. ,us, an animal model, even one PIBO. with reproducibility and demonstrable alterations in bio- markers, may not be useful for a study of PIBO if the final 11. Patient’s Perspectives histologic changes are lacking. It is likely that as other molecular tools, such as miRNA, One strength of this workshop is the engagement of the next-generation sequencing, or whole transcriptome se- affected families through the foundation “Stiftung Starke quencing, are perfected, we will be better able to establish the Lunge.” ,e families have been asked to give an overview of correct targets for study and, potentially, directed phar- their personal perspectives, especially their view on diag- macotherapy to either predict an increased risk for PIBO in nostic delay, treatment effectiveness, quality of life, and the risk patients or to treat those patients already developing impact on daily functioning. PIBO. ,e patient’s perspectives are shown in Table 4. ,e foundation “Stiftung Starke Lunge” has created a web page (http://www.starkelunge.de) dedicated to PIBO to inform 10.3. Microbiome. In recent years, the field of human and to support patients, their families, and medical pro- microbiome research has been growing exponentially in- fessionals. ,is German web page will be soon available in cluding the microbiome analysis of the respiratory system. It other languages. has been suggested that certain microbes play important roles in the development of healthy immune responses and 12. Future Directions that, on the other hand, microbial dysbiosis can contribute to chronic inflammatory lung diseases such as asthma, Postinfectious bronchiolitis obliterans (PIBO) in children is COPD, and CF [119]. Further studies presented data about a rare chronic irreversible obstructive pulmonary disease disordered or decreased microbial communities in asthma, that can follow any of several infections. ,e diagnosis is COPD, and CF [120–122]. Although microbiome data in made collectively by clinical, radiological, and physiological PIBO are scarce, studies of bronchiolitis obliterans syn- findings and a history suggestive of PIBO. However, there drome (BOS) demonstrated that both bacterial and viral are obvious differences in individual outcomes and the infections are associated with elevated risk of BOS and that natural course cannot be predicted. While some children are posttransplantation infections are associated with a worse left with serious structural and functional lung disease, outcome [123]. others experience more subtle effects. Further multicenter Usual procedures like microbiological culture, serol- prospective trials are required to understand the complexity ogy, point-of-care tests, or PCR can be successful but have of the disease and to define risk factors. Future effort should 12 Canadian Respiratory Journal focus on establishing supplemental specific diagnostic Respiratory Cell and Molecular Biology, vol. 51, no. 4, markers and ultimately a tailored approach of individual pp. 568–574, 2014. treatment options. [9] K. A. Hardy, D. V. Schidlow, and N. Zaeri, “Obliterative Better understanding of the entity PIBO will lead to bronchiolitis in children,” Chest, vol. 93, no. 3, pp. 460–466, better counselling of the affected patients and their families 1988. and will support these children and adolescents with a [10] A. U. Wells and R. M. du Bois, “Bronchiolitis in association with connective tissue disorders,” Clinics in Chest Medicine, future management plan for their lifelong respiratory vol. 14, no. 4, pp. 655–666, 1993. disease. [11] J. A. Cooper Jr., and R. A. Matthay, “Drug-induced pul- monary disease,” Disease-a-Month, vol. 33, no. 2, pp. 66–120, Disclosure 1987. [12] V. Poletti, G. Casoni, C. Ravaglia, M. Romagnoli, and ,e BO workshop 2017 was hosted by the foundation S. Tomassetti, “Bronchoalveolar lavage. Technical notes,” “Stiftung Starke Lunge” in cooperation with the Goethe Monaldi Archives for Chest Disease, vol. 75, no. 1, pp. 42-43, University Hospital Frankfurt, Germany. ,e foundation 2015. “Stiftung Starke Lunge” is a nonprofitable foundation [13] S. W. Crawford and J. G. Clark, “Bronchiolitis associated registered in Germany (Regierungspra¨sidium Darmstadt: with bone marrow transplantation,” Clinics in Chest Medi- AZ: I 1325d 04/11(6)-78) which supports research projects cine, vol. 14, no. 4, pp. 741–749, 1993. in pediatric rare lung diseases and specifically in bron- [14] G. S. Cheng, B. Storer, J. W. Chien et al., “Lung function chiolitis obliterans (BO) and brings together patients and trajectory in bronchiolitis obliterans syndrome after allo- the public with respiratory professionals to positively in- geneic hematopoietic cell transplant,” Annals of the Amer- ican Aoracic Society, vol. 13, no. 11, pp. 1932–1939, 2016. fluence lung health in pediatric rare lung diseases and BO. [15] S. P. Moonnumakal and L. L. Fan, “Bronchiolitis obliterans in children,” Current Opinion in Pediatrics, vol. 20, no. 3, Conflicts of Interest pp. 272–278, 2008. [16] S. Cazzato, V. Poletti, F. Bernardi et al., “Airway inflam- ,e authors declare that they have no conflicts of interest. mation and lung function decline in childhood post-infec- tious bronchiolitis obliterans,” Pediatric Pulmonology, Acknowledgments vol. 43, no. 4, pp. 381–390, 2008. [17] A. J. Colom, A. Maffey, F. Garcia Bournissen, and A. 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